The aim of this project is to determine how intentional mechanisms contribute to head stabilization in the pitch (sagittal) plane during voluntary trunk movements in persons with intact and bilaterally impaired vestibular (BVL) systems. One set of studies will quantify the contributions made by intentional mechanisms beyond reflex and mechanical processes. A second set of studies will evaluate what subtypes of intentional strategies are used to stabilize the head under different conditions. Empirical and computer modeling approaches will test the hypotheses that intentional mechanisms contribute significantly to head stabilization in the BVL as well as intact subject, and that these groups use different subtypes of strategies to stabilize their heads. In the empirical studies, subjects will sit on a sled that can translate in the anterior-posterior direction. They will voluntarily control translation of the sled by a hand-held joystick, and have some sled motions imposed on them. The sled translations generate forces that destabilize the head in the pitch plane. Analyses of head motion and torques and of electromyographic activity in the neck and trunk muscles will reveal how well subjects counteract the head-destabilizing forces during sled translation. The data also will reveal whether subjects stabilize their heads by avoiding the disturbance (reducing trunk subjects motion), anticipating and predictively cancelling the disturbance, by stiffening their neck through muscle cocontraction, or by relying on reflexes and passive mechanics. An analytical model of the head-neck control system will be used to run simulations that will determine how much intentional mechanisms contribute to head stabilization, and that will establish how combinations of intentional, mechanical and reflex strategies could work together to stabilize the head. This project is the first to evaluate intentional head-stabilizing mechanisms during voluntary trunk movements. The project also will add to basic knowledge of head control mechanisms by evaluating intentional processes during voluntary movements that are similar to activities of daily life, which generally involve voluntary rather than imposed motion of the trunk. The comparison of intact and BVL populations will elucidate why some persons with BVL recover relatively high functional levels, and others do not.